Upon reviewing the histologic sections from the primary tumors of 42\patients treated in the rhabdomyosarcoma (RMS) protocol at the NCI, we encountered 15 primitive round cell sarcomas of the soft tissue with no overt pattern, but with cytologic characteristics resembling those of an alveolar RMS. The referring diagnoses on those cases were RMS NOS, embryonal RMS or primitive sarcoma of soft tissues. All these cases clinically behaved in a fashion similar to that of alveolar RMS. For all the above reasons, we termed this primitive subtype of RMS, "solid variant" alveolar RMS. To evaluate the use of electron microscopy (EM) or immunocytochemistry in distinguishing this primitive RMS from other tumors, we reviewed the electron microscopic (EM) prints on 13 out of 15 cases and employed immunocytochemistry for muscle markers on 14 cases. The markers investigated were skeletal muscle myosin (MYS), myoglobin (NMG), creatine-phosphokinase (CPK) MM and desmin. The results showed that 13 out of 15 cases met the criteria for being diagnosed as RMS. The other 2 should be classified as primitive sarcomas NOS. Seven cases had cells with Z-band material and/or basal lamina and 3 inhibited a fair amount of cytoplasmic filaments. Two to 4 muscle markers were present in all RMS cases. Desmin was the most reliable single marker present in all cases. These immunocytochemical results represent extension of our previous work on RMS and support the myogenous nature of the solid variant alveolar RMS. They also suggest that EM and immunocytochemistry are useful tools to confirm the diagnosis of RMS in doubtful cases.